1- Polygon repairs damaged connections:
Damage of splines, keys, and serrations on shafts and in hubs often means scrapping expensive components. Most of the common repair methods involve removing the damage by machining, plugging bores and sleeving shafts, and remachining to the original condition. If you just reduce the shaft section and add a new spline, along with plugging and machining the mating bore to match, the smaller section may reduce strength below what was probably already insufficient. The best solution could be polygons.

Back in the end of the 80's/beginning of 90's, one of our customers in the west coast, had a hydraulic pump that needed its hydraulic motor shaft and its mating hub be replaced every year. The original design used a straight spline having 2.50" outside diameter, and parts were subjected to reversal torque, as they were oscillating within 270 degrees. Work was needed on this 12 in square drive hub with worn bore spline teeth and mating shaft. First, all spline teeth were removed by precision grinding the bore with a P3 polygon. Then a polygon plug was ground with press fit allowance of .002" to .003" and pressed into the hub bore. After pressing the plug into the hub, its bore was ground with a 1.75" P3 polygon to accept the mating ground polygon on the shaft. It was not necessary to grind off all the spline. The shorter length and reduced section of the polygon, was enough to take the load. The customer liked the performance of the polygon connection; no repair, no down time, no headache. And therefore, after about 18 month from the repair, they sent to us new (original) parts to replace the spline with a polygon connection, similar to the repaired unit.

2- Design change to polygon system bring success:
When tough applications result in repeatedly spline failures, the answer is polygons! That what some of our customers did. One of them said: "The polygon drives are dominant in the gearcase. The application is a difficult one, and involute splines in the same position have proven to be unsatisfactory in many instances where tough cutting is common."